Traditional electronic devices of the past, from toasters to television sets, performed a variety of functions, but normally had one thing in common--they didn't connect to any other electronic device.
Today's electronic devices are decidedly more complex. An increasing number of electronic devices today are interconnected to one or more other electronic devices. For example, personal computers are often connected to printers, tape drives, modems, etc., and communicate with these devices over a communications cable. This interconnectability of electronic devices has vastly expanded the function and power of electronic devices in general, but is not without its problems. One such problem caused by the interconnectability of electronic devices occurs when one device is powered-on while the other device is powered-off. This situation is commonly encountered when a powered-on computer is connected to a powered-off printer. When this situation occurs, the printer can be inadvertently powered-on through the communications cable. This can result in a variety of unpredictable and undesirable events occurring. One such undesirable event is the illumination of status lights and the false indication to a user that the printer is on and operational when it in fact is off. Other such undesirable events are more catastrophic--the burnout and failure of one or more components in the printer, or even in the computer itself--if power is fed back to the computer through another data line on the communications bus.
Previous attempts to solve the above problem have been wholly unsuccessful. Often the problem is simply ignored, with the blind hope that the powered-on device and the powered-off device can handle the situation without failing or giving an incorrect status indication to a user.